The topology of a block can be edited using the 'Drag face' tool which allows you to select a block surface and drag it along an axis, normal to the surface. To edit a block topology first go to the building level, select the block that you want to modify and click on the 'Drag face' tool. Then move the mouse cursor to the surface that you want to move. As you move the cursor across the block surfaces, each surface will highlight to help in selecting the required surface. When the mouse cursor is over the required surface, click to start dragging it:
Drag the surface using the mouse or key-in the distance to shift the surface by the required amount:
See also the Basic Drawing Tools 3 Tutorial